The invention relates to apparatuses for processing of substrates using radiofrequency induced plasma in a plasma chamber. In particular, the invention provides apparatuses and methods for generating a plasma of a uniform plasma density.
Gaseous plasma technology is a well known technique used for the fabrication of integrated circuits. Parallel plate reactors have been used extensively for exciting the gases in the reaction chamber to generate the chemical reactions required for thin film etching and deposition of wafers. In general, when coupling power through an insulator, previous hardware setups have used 13.56 MHz as the exciting frequency for the gases due to a higher excitation efficiency. For instance, see U.S. Pat. No. 4,948,458 ("Ogle"), the disclosure of which is hereby incorporated by reference.
In apparatuses such as that shown by Ogle, a radiofrequency magnetic field is induced in a low pressure reaction chamber by sending a radiofrequency resonant current through an external planar coil and passing the generated radiofrequency energy through a dielectric window in the chamber. The magnetic field generates a plasma by causing a circulating flux of electrons in a process gas introduced into the chamber to produce a region of ionic and radical species. The plasma so generated is used to etch or deposit materials on a wafer in the chamber.
It has been found that the plasma density across the surface area of the wafer is highly variable in such apparatuses, with densities measured across 150 mm and 200 mm wafer areas being as much as two times as great in some areas than in others. This non-uniform plasma density causes significantly non-uniform oxide and resist etch rates over measured wafer areas and makes it extremely difficult to control critical dimensions of fine line geometry on the wafer.
Normally, a flat dielectric window is used with the apparatuses. It has been observed that the magnetic flux of the planar coil is highest near the window center and, with a flat window, the induced electric field is consequently higher near the window center. The apparatuses and methods of the present invention utilize a dielectric window having a characteristic cross section, wherein the window is thicker at the center and thinner at the edges, to decrease the higher induced electric field near the window center.